1. Field of the Invention
This invention relates to a radiation image detector adapted for use in a radiation imaging apparatus, such as an X-ray imaging apparatus.
2. Description of the Related Art
With respect to radiation imaging operations for medical diagnoses, there have heretofore been known radiation imaging apparatuses, in which radiation image detectors (utilizing semiconductors as principal sections) are utilized for detecting radiation and converting the radiation into an electric signal. Ordinarily, the radiation image detectors may be classified into a direct conversion type, wherein the radiation is directly converted into electric charges, and wherein the thus formed electric charges are accumulated; and an indirect conversion type, wherein the radiation is converted into light by use of a scintillator, such as CsI:Tl or GOS (Gd2O2S:Tb), wherein the thus obtained light is then converted into electric charges by use of a photoconductive layer, and wherein the thus formed electric charges are accumulated. Also, in accordance with read-out techniques, the radiation image detectors may be classified into an optical read-out technique, wherein a read-out operation is performed with a radiation image detector utilizing a semiconductor material capable of generating the electric charges when being exposed to light; and a TFT technique, wherein the electric charges having been generated with the irradiation of the radiation are accumulated, and wherein the accumulated electric charges are read out through an operation, in which an electric switch, such as a thin film transistor (TFT), is turned on and off with respect to each of pixels.
The direct conversion types of the radiation image detectors are constituted for performing a radiation detecting operation, wherein a predetermined bias voltage is applied to a voltage applying electrode, which has been formed on a surface of a radiation-sensitive type of a semiconductor film (acting as a recording photoconductive layer), wherein the carriers having been generated with the irradiation of the radiation are collected by a carrier collecting electrode, which been formed on an opposite surface of the semiconductor film, wherein the thus collected carriers are accumulated at a charge accumulating region, and wherein the electric charges, which depend upon the electric charge quantity having been accumulated at the charge accumulating region, are taken out as a radiation detection signal. Ordinarily, the recording photoconductive layer is formed with amorphous selenium (a-Se) for its advantages of a high dark resistance and a high response speed.
Amorphous selenium is capable of easily coping with the formation of a layer having a large area by the utilization of a thin film forming technique, such as a vacuum evaporation technique. However, amorphous selenium is apt to contain many structure defects and is therefore apt to suffer from deterioration in sensitivity. Accordingly, ordinarily, in order for the performance to be enhanced, amorphous selenium is doped with appropriate quantities of impurities.
In, for example, U.S. Pat. No. 3,685,989, there is described a technique, wherein an appropriate photoconductive layer, through which both the electrons and the holes are capable of transiting, is obtained from processing for doping a-Se or an a-Se:As alloy with an alkali metal in a concentration falling within the range of 5 ppm to 5,000 ppm. However, in cases where a-Se is doped with Na in a concentration of at least 0.01 ppm, the problems are encountered in that interface crystallization is apt to occur at a contact interface with an electrode, in that image defects are apt to arise, in that the characteristics are apt to change due to moisture, and in that the durability is not capable of being kept long.
In order to solve the problems described above, the applicant has proposed a radiation image detector comprising an amorphous selenium layer, which is located as a crystallization preventing layer between a recording photoconductive layer and an electrode, and which contains at least one kind of element selected from the group consisting of As, Sb, and Bi. (The proposed radiation image detector is described in Japanese Unexamined Patent Publication No. 2007-213814.)
However, in cases where the crystallization preventing layer is located between the recording photoconductive layer and the electrode, which is located on the side opposite to a base plate, the new problems were encountered in that reticulations arise on a surface of the crystallization preventing layer with the passage of a long period of time (at least 100 hours) at a high temperature (40° C.). If the reticulations arise on the surface of the crystallization preventing layer, an artifact will occur in an obtained image. It is considered that the reticulations are caused to arise by a factor, such that a thermal expansion coefficient of the recording photoconductive layer (a-Se) is higher than the thermal expansion coefficient of the crystallization preventing layer, and by a factor, such that a modulus of elasticity of a-Se becomes markedly low at a glass transition temperature (40° C.) of a-Se, and resistance to the occurrence of the reticulations thus becomes low.